For the automation of mechanical milking, and particularly for the accurate control of the various milking parameters such as milking vacuum, pulsator vacuum, pulsator frequency and suction phase duration during the milking act or for ending the milking operation and automatic releasing of the milking equipment it is of essential importance that the milk flow be accurately measured all the time, so that control operations may be carried out in response to the momentous milk flow. It is of similar importance to accurately determine the total amount of milk obtained from the individual animal (cow, goat or ewe) in a milking act.
There are already known milk flow meters functioning on various physical principles. In the measuring of a milk flow the operating environment as well as the specific mechanical milking method result in particular conditions precluding the employ of the known flow meters. In particular, a milk flow meter should meet the following requirements:
1. The instrument should not interfere with the daily milking routine, i.e. it should be as compact as possible, lightweight and easy to handle, and in particular easy to clean, particularly in view of its employ in a chain-up stable.
2. The milk flow meter should be suitable for universal employ, i.e. it should be able to function properly in connection with overhead as well as underfloor milking lines under widely varying vacuum and pulsation conditions.
3. The measuring error, particularly with respect to the measuring of the daily total amount, should lie below 5%.
4. A milk flow meter of this type should be designed so that it can be cleaned without dismantling in the course of the daily rinsing of the milking equipment.
5. In a milk flow meter the operator-induced errors should be kept as small as possible, i.e. the device should be able to function substantially independent of its orientation, it should be of simple design and should be easy to handle.
Measuring of the milk flow has to be carried out at a location between the udder and the milk collecting line in which the milk of several animals is collected. Measuring of the milk flow is rendered relatively difficult by the fact that the milk to be measured presents itself at the measuring location in a varying and discontinuous two-phase stream. Thus the properties of the milk, for instance its viscosity, its electrical conductivity etc. vary not only from one animal to the next, but also for a single animal during a single milking act in accordance with the composition of the milk, such as its content of protein, fat or minerals. It is thus known for instance that the fat content of the milk increases towards the end of the milking act, so that the final amount has the highest fat content.
Moreover, as a substantially constant amount of atmospheric air is admitted to the milk discharge duct for aiding the flow of the milk therein, the relative air content of the milk also varies with the magnitude of the actual milk flow. Furthermore the amount of the admitted atmospheric air varies with the type of the milking equipment employed. The relative air content may also vary due to contamination or damage of the air inlet opening, so that in certain cases the amount of undesirable leaking air may be a multiple of the desired air amount. Furthermore, even if the relative air content is substantially constant, the mixing intensity between milk and air may vary widely. The mixing intensity may thus vary between a milk plug which is substantially free of air and a mixture of coarse or fine foam and microscopic air bubbles. A further difficulty in measuring the milk flow results from the fact that the milk flow pulsates more or less strongly and rather irregularly at the measuring location due to the particularities of the mechanical milking method. Further difficulties in the milk flow measuring art result from the fact that the flow velocity of the milk at the measuring location is the product of several variable factors such as the momentous vacuum magnitude, the flow amount, the outer and inner friction of the milk or the conveyance level. Finally, the flow measurement has to be carried out under milking vacuum conditions without disturbing the vacuum.
Already known from German Offenlegungs Schrifts Nos. 28 10 376 and 28 39 101 are milk amount measuring devices, wherein the milk is tangentially introduced into an upper milk collecting chamber located above a measuring chamber communicating with a milk discharge duct at its lower end. By periodically opening a flow connection between the collecting chamber and the measuring chamber and simultaneously closing the discharge duct, milk is periodically introduced from the collecting chamber into the measuring chamber, wherein the filling height is measured by means of a float arrangement, whereupon the flow connection between the collecting chamber and the measuring chamber is closed and the discharge duct is opened to discharge the previously measured amount. If this device assumes an inclined position, there is the danger of the movable parts getting jammed. The measuring accuracy of the device depends largely on its deviation from the vertical orientation. Moreover, the measurements can only be taken at certain intervals, i.e. a continuous measurement of the milk flow is not possible.
The institute for agricultural technology at the technical university of Munchen-Weihenstephan has developed a ring electrode milk flow meter wherein the milk flows through a vertically oriented tube, the upper end of which is enlarged to form a milk collecting chamber whereinto the milk is introduced in a tangential direction. The lower cylindrical portion of the measuring tube contains two spaced annular electrodes between which the electrical resistance of the milk body actually extending between the two electrodes is measured. Measuring of the milk flow on the basis of its electric conductivity offers severe problems, however. Thus the conductivity of the milk varies with the relative air content, the composition, i.e. the relative protein, fat and anorganic matter content of the milk or even with the temperature of the milk. Furthermore the flow velocity within the measuring tube depends on the flow amount, so that measuring of the milk amount is not without problems. In addition, it is scarcely possible to obtain accurate measurements in the lower flow range (less than 1 l/min), if the device is designed for a maximum flow of about 6 l/min, this being the minimum requirement for a modern high-production cow.
Known from U.S. Pat. No. 4,122,718 is a device for measuring the liquid level in a liquid contained in a container. Two electrodes embedded in a plastics material are dipped into the container, the filling level of which is to be measured. An AC voltage applied to the electrodes is employed for measuring the capacitance between the two electrodes, which varies with the filling level of the container.
Known from U.S. Pat. No. 4,173,892 is a similar device for measuring the total amount of milk obtained from a single animal in one milking act. In this device, the milk flows into a collecting vessel having two electrodes located opposite one another on its inner and outer surfaces. The capacity between the two electrodes is measured with the aid of an AC voltage and varies with the filling level of the vessel. This known device only permits the total milk amount obtained in one milking act to be measured. The accuracy of the measurement is considerably affected by the foam floating on the surface of the milk.
It is an object of the present invention to devise a milk flow meter permitting the flow of milk to be measured continuously and as accurately as possible.